A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To be most effective, the lithographic apparatus is used in as clean an environment as possible. One of the main reasons for using a clean environment is to prevent contamination of the substrate and any optical surfaces which are used to manipulate radiation beams used to apply a desired pattern onto the substrate. For example, a lithographic apparatus using an extreme ultraviolet (EUV) radiation beam is known to generate contaminants which can lead to a deposit forming on the optical surfaces. For example, irradiation of some optical surfaces with EUV is known to cause the build up of a carbonaceous deposit on these optical surfaces. These deposits may reduce the operating resolution of the lithographic apparatus. It is thus desirable to minimize the contamination of optical surfaces and, when necessary, clean the surfaces to remove the deposits. Cleaning of the optical surfaces is undertaken when the level of the contaminant is such that the operation of the lithographic apparatus is compromised. Therefore, it is desirable to be able to detect the level of contaminants on the optical surfaces.